Liquid crystal displays have been widely used for monitor for personal computer and cellular phone, television, etc. because they are advantageous in that they can operate at low voltage with low power consumption and are available in small size and thickness. These liquid crystal displays have been proposed in various modes depending on the alignment of liquid crystal molecules in the liquid crystal cell. To date, TN mode, in which liquid crystal molecules are aligned twisted at about 90 degrees from the lower substrate to the upper substrate of the liquid crystal cell, has been a mainstream.
A liquid crystal display normally comprises a liquid crystal cell, an optical compensation sheet and a polarizer. The optical compensation sheet is used to eliminate undesirable coloring of image or expand the viewing angle. As such an optical compensation sheet there is used a stretched birefringent film or a transparent film coated with a liquid crystal. For example, Japanese Patent No. 2587398 discloses a technique for the expansion of the viewing angle involving the application to a TN mode liquid crystal cell of an optical compensation sheet obtained by spreading a discotic liquid crystal over a triacetyl cellulose film, and then orienting and fixing the coat layer. However, liquid crystal displays for TV use which are supposed to give a wide screen image that can be viewed at various angles have severe requirements for dependence on viewing angle. These requirements cannot be met even by the aforementioned approach. To this end, liquid crystal displays of modes different from TN mode, including IPS (In-Plane Switching) mode, OCB (Optically Compensatory Bend) mode, VA (Vertically Aligned) mode, have been under study. In particular, VA mode has been noted as liquid crystal display for TV use because it gives a high contrast image and can be produced in a relatively high yield.
A cellulose acylate film is normally characterized by a higher optical isotropy (lower retardation value) than other polymer films. Accordingly, it is normally practiced to use a cellulose acetate film in uses requiring optical isotropy such as polarizing plate.
On the contrary, the optical compensation sheet (retardation film) for liquid crystal display is required to have optical anisotropy (high retardation value). In particular, the optical compensation sheet for VA mode is required to have an in-plane retardation (Re) of from 30 to 200 nm and a thickness direction retardation (Rth) of from 70 to 400 nm. Accordingly, it has been usually practiced to use, as an optical compensation sheet, a synthetic polymer film having a high retardation value such as polycarbonate film and polysulfone film. As mentioned above, it was an ordinary principle in the art of optical materials that when the polymer film is required to have optical anisotropy (high retardation value), a synthetic polymer film is used, while the polymer film is required to have optical isotropy (low retardation value), a cellulose acetate film is used.
JP-A-2001-249223 overthrows this conventional general principle and proposes a cellulose acylate film having a high retardation value that can be used also for purposes requiring optical anisotropy. In accordance with this proposal, an aromatic compound having at least two aromatic rings, particularly a compound having 1,3,5-triazine ring, is added to cellulose triacetate to be stretched in order to realize a cellulose triacetate film having a high retardation value. It is generally known that a cellulose triacetate is a polymer material that can be difficulty stretched and provided with a high birefringence. However, JP-A-2001-249223 proposes that when additives are oriented at the same time with stretching, making it possible to raise birefringence and realize a high retardation value. This film is advantageous in that it can act also as a protective layer for polarizing plate and thus can provide an inexpensive thin liquid crystal display.
It is disclosed that the use of an optically compensatory film composed of only a cellulose acetate film having an Re value and an Rth value falling within the defined in JP-A-2001-249223 (20 nm to 70 nm and 70 nm to 400 nm, respectively) makes it possible to obtain a wide viewing angle. However, no reference is made to means of reducing the difference in tint (color shift) between in the direction normal to the liquid crystal cell and in other oblique directions at the black state.
In recent years, in the art of liquid crystal displays, there has been a growing demand for the improvement of not only the viewing angle defined by contrast ratio but also the display color.